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Astron. Astrophys. 336, 840-854 (1998)

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Stellar disks of optically flocculent and grand design spirals *

Decoupling of stellar and gaseous disks

P.J. Grosbol 1 and P.A. Patsis 2, 3

1 European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching, Germany
2 Max-Planck Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
3 Astronomisches Rechen Institut, Mönchhofstraße 12-14, D-69120 Heidelberg, Germany

Received 24 March 1998 / Accepted 8 June 1998


Accurate surface photometry of five spiral galaxies (NGC 3223, NGC 5085, NGC 5247, NGC 5861 and NGC 7083) in visual and near-infrared bands is presented. All galaxies show grand design spiral structure in K[FORMULA] although several have a flocculent appearance on blue images suggesting that stellar and gaseous disks are decoupled. The decomposition of the K[FORMULA] maps in axisymmetric components suggests that both a spherical bulge and a flat exponential disk are required to explain the light distribution in the bulge regions. The central disk has a short scale length and sky projection parameters similar to the main disk. The scale length of the main exponential disk measured in the interarm region is consistently smaller than the value determined from the arms for three of the galaxies. Weak oval distortions were found in three galaxies while only one was classified as such. The galaxies with oval distortions have less concentrated and fainter bulges than those in the two galaxies without such ovals.

Model rotation curves were constructed for each galaxy based on the axisymmetric decomposition of their K[FORMULA] surface brightness maps and observed velocity data. The mass-to-light ratio estimated in K[FORMULA] for the disk component was around 0.7 in solar units for all galaxies using the disk scale length derived from the interarm regions.

Pitch angles of the main two-armed spiral estimated in B,V,I and K[FORMULA] show a systematic trend of arms being tighter in bluer colors which suggests the presence of density waves in the galaxies. Comparing the extent of the spiral pattern in the galaxies with their angular velocity curves, the best agreement was obtained when the symmetric, two-armed spiral starts just outside ILR and terminates around the 4:1 resonance. The spirals continue beyond this region but are weaker and more fragmented. The pattern speed estimated for the two Sb galaxies was significantly higher than that for the Sc galaxies.

Several galaxies have dust spirals inside the main stellar spiral. In most cases, the main spiral pattern is more open in the inner region than further out where it is well approximated with a logarithmic spiral. This may support a scenario where a central oval distortion drives the spiral, which then would correspond to the long waved solution of the dispersion relation.

Key words: galaxies: kinematics and dynamics – galaxies: photometry – galaxies: spiral – galaxies: structure – infrared: galaxies

* Based on observations collected at the European Southern Observatory, La Silla, Chile

Send offprint requests to: P.J. Grosbol,(pgrosbol@eso.org)

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© European Southern Observatory (ESO) 1998

Online publication: July 27, 1998